A large number of extractants are known that are capable of extracting zinc from solutions that may also contain one or more other metals such as iron, copper, silver, cadmium, cobalt, nickel, manganese, magnesium, calcium, sodium, potassium, arsenic and antimony. Zinc and other metals have been extracted from solutions with organo phosphorus compounds such as substituted organic phosphoric acids, particularly di-2 ethylhexylphosphoric acid (D2EHPA). Methods for the extraction of zinc with D2EHPA are disclosed in references that include Hydrometallurgy 3 (4), 327-342, Oct. 1978; Chem. Engg. Res. and Design 61, 62-66, Jan. 1983; U.S. Pat. Nos. 3 989 607, 3 441 372, 4 124 462, 4 423 012, 4 552 629 and 4 618 428; and CA Patents 1 083 830, 1 098 719 and 1 198 290.
It is also known that zinc and many other metals form complexes with substituted phosphinic acids. Methods that use substituted phosphinic acids for the extraction of zinc from solutions are disclosed, for example, in U.S. Pat. Nos. 3 966 569 and 4 721 605. According to the former, zinc and iron are removed with a dialkylphosphinic acid in a multiple-stage solvent extraction process using different extractants. According to the latter, a metal from a first group consisting of zinc, silver, cadmium, mercury, nickel, cobalt and copper can be extracted from solution additionally containing a metal from a second group consisting of calcium and magnesium by contacting an aqueous solution containing one or more of the metals of the first group with an organic-soluble substituted dithiophosphinic acid or salt thereof at a pH of from 0.2 to 3.0. It is apparent from the disclosure of this patent that, when more than one metal of the first-recited group is present in the solution, all of these metals are extracted. The extraction is only selective with respect to the metals of the second-recited group.
None of these references disclose the selective extraction of zinc from a zinc sulfate solution with an extractant mixture that contains both an organic substituted phosphoric acid and an organic substituted thiophosphinic acid.